High-strength, heat-insulating and spalling-resistant composite mortars for post-fire tunnel lining repair

With the increasing scale of tunnels and growing traffic volume, fire has become one of the most critical issues for tunnel operation safety. Under fire exposure, tunnel lining concrete is highly susceptible to rapid strength degradation, cracking and explosive spalling, while conventional repair materials struggle to simultaneously satisfy the requirements of mechanical strength, thermal insulation and spalling resistance. In this study, a composite mortar is developed for post-fire tunnel lining repair and hydrogels are innovatively incorporated to improve its fire resistance. The effects of Hydrogel A (aramid nanofiber hydrogel) and Hydrogel B (PVA/PAA/MMT/Fe 3+ composite hydrogel) on density, compressive strength, thermal conductivity and high-temperature spalling resistance are investigated. Subsequently, a multi-component composite mortar containing hydrogels, glazed hollow beads (GHBs), PVA fibers and basalt fibers is optimized through orthogonal experimental design. The performance of the optimal composite mortar is validated by fire exposure tests on concrete slabs, while its microstructural characteristics are revealed through SEM and X-CT analyses. The results show that incorporating 0.8% Hydrogel A alone reduces thermal conductivity by 26%, while Hydrogel B exhibits a beneficial effect on spalling resistance. The optimal mix proportion consists of 0.8% Hydrogel A, 0.8% Hydrogel B, 60% GHBs, 0.2% PVA fibers and 0.8% basalt fibers, achieving a compressive strength of 52.5 MPa, a thermal conductivity of 0.2566 W/(m·K) and no spalling under fire exposure. The enhanced performance of the composite mortar is attributed to the pore network created by hydrogels, the closed-pore insulating structure provided by GHBs and the inhibition of crack propagation by fibers, offering an optimized balance between strength, thermal insulation and high-temperature spalling resistance.